Shape memory alloys such as Ti-Ni (titanium-nickel) alloy and Cu-Zn-Al (copper-zinc-aluminium) alloy have specific nature that if the deformed alloy is heated greater than a certain temperature (phase transfer temperature), the alloy takes the original shape previously memorized. In other words, the shape memory alloy constitutes a temperature sensor and has a heat-energy/mechanical-energy conversion function.
Paying attention to such function, a study is being made, e.g., on an actuator for robot arms requiring positional precision, wherein a shape memory alloy worked in the form of a rod or tape is used as the actuator and a drive force therefor is obtained by controlling the expansion and contraction of the actuator while heating and cooling. In a conventional shape memory alloy actuator of this type, the ends of the alloy material of a rod or tape shape are coupled to a driven member through a biasing spring having a preset tension. The alloy material is then rapidly heated or cooled by heating means or cooling means. The heating means is generally of a resistor heating type wherein Joule heat is used, the heat being generaged while an electric current flows in the alloy. As the cooling means, there is known a forced air cooling whereby the shape memory alloy wire is always subject to an air flow. However, with the this forced air cooling, the cooled air becomes a load during heating so that there is a limit of deformation response time. To obviate this limit, there have been studied many methods, e.g., a method wherein forced air cooling is effected only during cooling and it is stopped during heating, a method using water cooling instead of forced air cooling, and so on.
Despite the above, the actuator system using a shape memory alloy has an ample possibility of achieving substantially miniaturizing and eight-weighting the system and simplifying the mechanism, as compared with a conventional general actuator system constructed of a combination of temperature sensors, control units, motors and the like.
However, in the conventional shape memory alloy actuator, the shape memory alloy wire or tape itself constituting the actuator has been made of a shape memory alloy of a single continuous form. Thus, the following problems exist:
(a) The wire length of an actuator is detected by a sensor at all times to control the outputs of the heating and cooling means based on a feedback signal from the sensor. The need of such a closed loop control makes the system control complicated.
(b) The response property of the system control is poor (particularly, a low response speed during cooling), and it takes a time for the shape memory alloy actuator to be changed in a certain form.
(c) Since the response property varies depending upon which portion of the shape memory alloy is to be subjected to air flow from a heating source or a cooling source, or which portion is to be contacted with the heat source or the cooling source, control precision becomes bad.